I had a similar issue with stamina regeneration until the merchant in riverwood sold me one.
as you level up, the amount of possible modifiers increases so your odds of seeing any single modifier might actually decrease because youre drawing from a larger pool. modifiers should not disappear after hitting a certain level so keep searching.
I'm level 77 and i have still yet to find a boots of muffling.
That's the issue here. I'm thinking that there might be some bug and that certain items (In this case, Resist Poison and Boots of Muffling) stop appearing after you've reached a certain level. I've only been searching for these items at or near the level cap and they haven't appeared. Doesn't matter if I reset a merchant 200 times or if I travel around looking in multiple merchant inventories.
This wouldn't be the first time that being at the level cap has harmed me. Being at the level cap also prevents "The Wolf Queen Awakened" from being initialized because the trigger for the Courier is to level up (Something which you can't do when you're at the level cap ... >.< ). I've been looking this whole time for boots of Muffling or Necklace of Poison Resistance/Immunity and I've got zilch. Nothing. Nada. *Shrugs*